This article will have all you need to know about the Biology IAA coursework on the investigation into the effect of light intensity on the rate of photosynthesis. Yes, another boring IAA exam which makes it vital to have no slip ups in the method, validity, reliability, evaluation or conclusion.

Independent variable causes a change in Dependent Variable and it isn't possible that Dependent Variable could cause a change in Independent Variable. So, the light intensity causes a change to the rate of photosynthesis.

Controlled VariablesAmount of background light, volume of indicator, carbon dioxide concentration in water, temperature of water, lamp (bulb), colour of indicator at start, length of pondweed, species of pondweed, concentration of indicator, healthiness of pondweed and observer.

Equipment List

Three test tubes.

Healthy Pondweed plant (still alive in water).

Glass container for remaining unused pondweed to be in.

Hydrogencarbonate indicator.

Scissors (to cut pondweed with).

Three rubber bungs.

A sheet of aluminium.

A lamp.

30cm ruler.

Stopwatch.

A purple spot diagram.

Method1) Label three tubes 1-3 to help stop contamination.2) Cut 6 lengths of pondweed at 10cm long (do this while the pondweed is underwater) and place two in each test tube.3) Pour 25ml of hydrogencarbonate indicator into all three test tubes which should be enough to cover the pondweed.4) Close each tube with a rubber bung making sure it's air tight.5) Cover tube 1 completely with aluminium foil to exclude light.6) Place tube 2 directly next to a light source (such as a lamp) and then tube 3 directly 10cm away from the light source.7) Measure the time taken for the indicator to change to purple using the 'purple spot diagram' to decide this.8) Repeat steps 1-7.9) Ignore any anomalous data and repeat to compensate for this.

The graph produced from this experiment isn't very useful as it will only have 3 points on it making it harder to see a pattern or any type of correlation. However, this is what the table should look like:

Pattern From The Table

From the table, it is clear that light intensity on the pondweed effects the time taken for the hydrogen carbonate indicator to go from red to purple as high light intensity produced an average of 27.8 minutes where low intensity produced an average of 35 minutes. This shows the higher the light intensity, the less time it takes for the indicator to go purple. Where there is no light, there is no photosynthesis so no change.

The method could have been made more valid by (The point/Explanation for the point)

Keeping the temperature the same using a sheet of glass because temperature effects the rate of photosynthesis.

Use one lamp instead of two as the light source as the two different lamps could have had two different light intensity meaning two different rates of photosynthesis.

Using same amount indicator or use a pH probe as different amounts will produce different shades of colours making it more difficult to establish when the colour has gone purple.

Using same length of pondweed as the different lengths of pondweed (in mm) would have used slightly different amounts of carbon dioxide.

The method was made reliable by..

Labelling the tubes to stop contamination.

Cutting the pondweed underwater.

Close each test tube with a rubber bung.

Repeating the experiment.

Ignoring any anomalous data and repeat to compensate.

Conclusion (this is where you should use your results and scientific knowledge)Ultimately, the higher the light intensity the less time it takes for the indicator to turn purple. This is because light effects the rate of photosynthesis. Therefore, the more light, the faster the pondweed will photosynthesis. The change in carbon dioxide (due to pondweed photosynthesising) dissolved in the water effects the acidity of the water: the more carbon dioxide dissolved the more acidic the water will be. As the carbon dioxide in the water has decreased, the acidity of the water decreasescausing the indicator to change from red to purple. This proves as light increases, so does rate of photosynthesis which is because the chlorophyll has more light to absorb for photosynthesis.UpdateFrom the comments below, there seems to be some confusion about what exact indicator is used in this experiment. Hydrogencarbonate indicator (also known as biocarbonate indicator) is used because it is a pH indicator that is sensitive enough to display a colour change when the levels of carbon dioxide changes. When the levels of carbon dioxide decrease, the acidity of the water ever so slightly decreases which the hydrogencarbonate indicator reacts to changing colour from red to purple. Therefore, to sum up:

25ml of hydrogencarbonate indicator is poured into the test tubes with pondweed in them.

The pondweed photosynthesises decreasing the level of CO2 in the water.

The decreased level of CO2 in the water causes the acidity of the water to decrease.

The hydrogencarbonate changes colour from red to purple due to the change in acidity.

The 'purple spot diagram' is a simple diagram with different shades of purple on it. This helps making the test valid because you will be able to stop the clock at the same specific shade of purple for each test tube of pondweed.

About Will Green

A student in England studying Automotive Engineering with Motorsport, Will created Ask Will Online back in 2010 to help students revise and bloggers make money. You can follow AskWillOnline via @AskWillOnline.

137
comments so far:

The highlighting is meant to make clear the different sections of an answer. For example the conclusion which has the answer where you use your results and then scientific knowledge to back up your findings.

Well using the notes above will really boost your chances of gaining a high grade. I used these exact notes and achieved an A*. If you revise and learn them off by heart, you should be able gain an easy A* too.

The IAA GCSE exam i did basically asked for the method of the experiment above with questions such as how was the method reliable, valid and accurate. There was nothing about the table. Even though, it's still important to remeber the averages in the table and understand what they mean.

Basically the pondweed is in water. When the pondweed starts photosynthesising, it causes the plant to start taking in oxygen from the water and give off carbon dioxide (you will see the carbon dioxide as bubbles).

Fact: Carbon dioxide makes water slightly more acidic. Therefore, the more CO2 in water the more acidic it will be. The less CO2 in water, the less acidic it will be (more alkali but this is only by around 0.5pH)

Because the Pondweed is giving on CO2 into the water, it is making the water slightly more acidic. This causes the indicator to change colour from orange to red (the indicator goes red when water gets more acidic).

I hope you understand and this helps. Any more questions feel free to ask.

I didn't get any past papers. This is because every past paper has a different experiment to do on it so you can't really do it.

However, I can help you out with what I remember from the layout of the test.

There are two parts, both on experiments:

First part is on the experiment you did.Second part is on another experiment similar to yours that some kids did.

You are asked questions like 'What is the method for this experiment?' 'How did you make the test reliable/accurate/valid?' and 'How could you improve the experiment if you did it again' Which answer would be to repeat experiment more times, use more accurate equipment etc...

The second part of the test you can't really revise for as you don't know what the experiment is about. It is similar to the first section which is always helpful. Hope this helps you out.

You will probably find this page quite useful, it has past papers of other exams you will have to do such as multiple choice etc...

If the question asks you to write out the full method to the experiement, then yes, write out the apparatus you will be using before you start on the method. You might get a question literally asking for what appartus you used in the exam: it varies. Include as much detail as possibly EVERYWHERE. Just think of it in this way, give the examiner and excuse to give you a mark (by writing loads of detail).

I just pictured the experiment out in my head. To be honest, there's not much you need to learn. All you need to know is the method: once you have the method locked up in your memory, everything else will come naturally when you answer the questions.

Distilled water is one of the cleanest types of water. By rinsing the test tubes keeps the experiment accurate as if you didn't rinse the test tubes before you do the experiment again, bits of indictator, pondweed etc.. might have got stuck in the test tube causing the results to be inaccurate (anomalous). Distilled water (purified by distillation) causes all the impurities to be removed so that when you rinse whatever your rinsing with distilled water, it won't react in any way at all.

Well, I don't think you need to know that plants respire in dark (it won't come up in the exam). An example of that type of experiment would be something something lik putting test tube in a fridge, on the side and in an incubator (which can control temperature). You could say about this exam it is not valid as the light intensity for each tube changes when only temperature should change.

Dude thatn you SOOOOO MUCH u do not knnow how much this helped me I LOVE YOU!! haha so listen did u do the hypothesis, cuz we have to do a lab report about it and have to include scientific explanation what should i write. please helpp!!!

Your welcome! By the way, what's causing this to get loads of comments? Have people got exams in this coming up? To the next comment, hypothesis? I don't know what that experiment was but the ones that are currently on Ask Will Online are:- Light Intensity Effecting Rate of Photosynthesis.- Rate of Reaction Between Sodium Sulphate and Hydrochloric Acid.- Height of Holly Leaves Affect Prickliness.- Investigating Human Power.- Using a light source, LDR and tracing paper to model absorption of gamma rays by different thickness of material.

yes its the light intensity effecting the rate of photosynthesis. i mean hypothesis is when u predict for example i think that the higher the lite intensity the higher the rate of photosynthesis, but what i cant answer is why?using scientific explanation

To the next comment, I'll try and explain how the higher the light intensity the the higher the rate of photosynthesis. The molecules in plants that cause the plant to photosynthesise are called chlorophyll and they are what give plants the green look. When light is present on them, they photosyntheisise using carbohydrates the plant has stored to use carbon dioxide to grow with the waste product being oxygen. When light intensity increases, the rate at which the chlorophyll changes CO2 to O2 increases too as the chlorophyll are getting more energy from the light. However, after a certain amount of light intensity, photosynthesis stays constant because each chlorophyll molecule can only take in a certain amount of light meaning any extra light will be wasted.

What in the world is a 'purple spot indicator' and how do you measure the time taken to change in the one covered in aluminum foil... you shouldn't be able to see anything or else you would expose it to light.

Where and when would you use a purple spot diagram and indicator? Sorry, I don't know what that is.

However, you will have to peel back the aluminium foil to see if the colour therefore reaction has occurred. You can then argue this makes the experiment invalid as you are giving the pondweed light to photosynthesise. That is the only way to see if the reaction has occurred or not.

Oh I get it now, the purple spot diagram is a measure of different shades of purple. This helps you decide when the water goes the 'right shade' of purple as you will be able to see on the indicator the 'right shade' of purple you want to measure the experiment with.

What you want to do is stick to one shade of purple on the chart. When you think the water colour has changed to that shade of purple, you then record the results. In the evaluation of the experiment, you can say how this made the test more invalid as you didn't record the results evertime the water colour went the exact shade of purple you measured it to on the chart.

I find that last comment difficult to understand. What part of this article is wrong? Because these notes I have provided for others gained me and fellow friends an A* at GCSE in Biology IAA experiments.

It's alright! For your question, if the results are similar to above, use a bar chart. If the results are lets say temperature against time for indicator to go red, its a line graph. If you have lots of points, the rule of thumb is line. Few points, in this case three variables, its bar chart.

Basically the pondweed is in water. When the pondweed starts photosynthesising, it causes the plant to start taking in oxygen from the water and give off carbon dioxide (you will see the carbon dioxide as bubbles).

Fact: Carbon dioxide makes water slightly more acidic. Therefore, the more CO2 in water the more acidic it will be. The less CO2 in water, the less acidic it will be (more alkali but this is only by around 0.5pH)

Because the Pondweed is giving on CO2 into the water, it is making the water slightly more acidic. This causes the indicator to change colour from orange to red (the indicator goes red when water gets more acidic).

I hope you understand and this helps. Any more questions feel free to ask.

The purple spot diagram helps you decide when the water has gone red enough to record the result. It shows a chart of different pHs forgot the red of an acid to the purple of the alkali. It helps to measure the experiment to one shade of red and not different shades of red.

The equipment list is extremely easy to do. Go through the method and picture doing the experiment. Write down all the things you will need to do the experiment.

apparently there are 2 methods to write in the test? can you recall what they are? and also .. there are 2 graphs to plot, one a bar and the other one is something quite difficult to plot? do you remember? THANKS (:

Sorry, I can't really remember. I didn't have to plot two graphs though not even one. The main part of this article you should learn is the method as 99/100, they will ask you to write it out for this experiment.

There is only one colour being purple/red. It's the shade of red that matters, lighter shades (orange) mean there is CO2 in the water. As the CO2 decreases in water, the colour becomes more vibrantly red.

Yes, I'm sorry if I make myself confusing but it's difficult answering everyone's questions! They are the opposite of humans. To respire, we use oxygen and the waste product is CO2. To respire, plants take in CO2 which makes their waste product O2.

You said that "to respire, plants take in CO2 which makes their waste product O2." This isn't true though - plant cells repire just like animal cells but plants can also photosynthesize which is where they convert CO2 to O2 and glucose. A plant in the dark will still respire - using O2 and releasing CO2 but it will not photosynthesize.

it says on my candidate research notes you have to write a hypothesis, research sources and method of which i picked yours but further at the bottom it says context i have done this for a different one but i need ideas for this one please.

I might just be being extremely dumb here but i'm confused about which colour the indicator is meant to go because in the method it says purple but on the table it says red... thank you for doing this though because it has helped me SO much :)

We are measuring the change in acidity of the water due to the pondweed photosynthesising. As the CO2 levels decrease in the water, the acidity decreases too which causes a change in colour to the hydrogencarbonate indicator.

I did OCR so cannot gurantee if that is OK or not. My gut feeling is if you have used the website as research make sure they are in your own words and not completely copied. You should be fine if you do that.

When looking at the safety of this experiment, a risk would be that the lamp's bulb will get extremely hot due to it being a halogen bulb. Therefore, make sure that you do not touch the bulb and only move the lamp via it's base. Another potential hazard is the water around the electricity from the lamp - make sure you keep them apart. From this, the risk assessment is quite low but it should not be ignored.

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About Will Green

At first, I was using Ask Will Online to help myself revise for school exams. I then started to find areas in blogging that were tricky to overcome so I started making articles on generally blogging help. Now, Ask Will Online has differentiated into many sub categories so that even more people can now take use of my website.